1. Field of the Invention
The present invention relates to a particle sensor for detecting the solid particles contained in a fluid.
2. Description of Related Art
When solid particles are present in a fluid (a liquid or a gas), it is necessary in some cases to detect the presence of the particles. Detection of such particles is particularly important when the particles present in the fluid badly affect the intended action of the fluid.
Lubricating oils (e.g. engine oils) are used in internal combustion engines of automobiles or heavy machinery in order to reduce the frictional resistance and abrasion of the rotating surfaces and sliding surfaces of the engines. Operation of such internal combustion engines generates particles (e.g. metal fine particles) caused by abrasion. The particles are taken into the lubricating oils and may accelerate the abrasion of the rotating surfaces and sliding surfaces. The particles present in the lubricating oils are ordinarily removed by the use of a filter such as oil filter or the like. The condition of the lubricating oils can be monitored in detail by detecting the particles present in the lubricating oils.
Hydraulic oils and flushing oils are used in power transmission mechanisms (e.g. transmission), oil hydraulic pipe systems (e.g. oil hydraulic servo valve), rolling, pressing, etc. It is important to control the condition of these oils by detecting the solid particles present in the oils and/or examining the viscosities of the oils.
It is also important to detect the particles suspended in the air or examine their concentration in the air in order to monitor the level of air pollution. Such detection includes detection of the particles suspended in the exhaust gas discharged from a plant, a factory or the like.
As a sensor for detecting particles contained in a fluid, there is disclosed a particle sensor comprising a sensor element using a piezoelectric film, in Japanese Patent Application Laid-Open (Kokai) No. 7-301594. With this sensor, particles present in a fluid collide with the detecting section (having a piezoelectric film) of the sensor element or with the vibrating section (mounting the detecting section thereon) of the sensor element. As a result the vibrating section and the detecting section are vibrated, and the piezoelectric film converts the vibration into electric signals. These electric signals are outputted by electrodes holding the piezoelectric film between them.
In order to maintain the condition of a fluid at an intended level by, for example, conducting filtration as necessary, it is useful to know information not only on the presence or concentration of particles in the fluid but also on the sizes or particle size distribution of these particles. In detecting particles present in a fluid using the above-mentioned conventional particle sensor, the speed of the fluid hitting the sensor element of the sensor is set at a given constant level and, in this state, the amplitude (voltage) of each signal generated by the collision of particles in the fluid, with the sensor element is measured to determine the particle sizes.
The measured values of amplitude of each signal, however, show high scattering and have low accuracy. Increase in times of measurement for higher accuracy in turn increases the complexity of signal processing, and also requires a longer time for measurement.